Bone marrow contains the cells with osteogenic potential. The ability to identify, isolate and manipulate cells within the osteoprogenitor lineage pathway is an essential component of a modern strategy for the somatic gene therapy. The specific aim of this project is to develop a model of lineage progression that can isolate an intact viable subpopulation of cells at specific stage of differentiation. The model is based on the hypothesis that a specific collagen promoter fragment activates in an off-on manner at early stage of osteoblast differentiation, well before the off-on activation of transgenes that should appear during early (bone sialoprotein) and late mineralization (osteocalcin). We will use transgenic mice harboring bone expressed promoters driving easily recognized marker genes. If we can show that the existing transgenes function as markers for identifying cells at different stages along the pathway, they could be identified by supravital fluorescent stains allowing a relatively homogenous subpopulation of cells to be isolated for molecular analysis and to be followed for their subsequent fate both in culture and in intact animals. This analysis will form the basis for formulating a strategy for genetic manipulation of a population of cells capable of entering into the bone lineage.